1. Field of the Invention
This invention relates to a driving circuit for an insulated gate bipolar transistor (hereinafter referred to as IGBT) which drives the gate of the IGBT safely without accompanying overcurrent phenomena.
2. Prior Art
Many types of power converters which convert d.c. to a.c. or d.c. to d.c. using IGBT elements have been proposed. This IGBT has an insulated gate and is operated in a bipolar mode. The IGBT has features of short switching time and low ON voltage. Therefore this element enables miniaturization, low cost and high frequency control of high power which cannot be performed by conventional bipolar transistors or MOS FETs.
FIG. 1 is a circuit diagram of a fundamental chopper using an IGBT. According to FIG. 1, an IGBT 1 and a load 3 are connected in series to both terminals of a d.c. power source 2. Power is supplied to the load 3 by on-off controlling the IGBT 1.
In order to achieve on-off control of the IGBT 1, serially connected power sources 4 and 5 for gate and serially connected NPN transistor 6 and PNP transistor 7 are connected in parallel, and the common connection point of transistors 6 and 7 is connected to the gate terminal of IGBT 1. Moreover, the bases of transistors 6 and 7 are commonly connected to an input terminal 8 for the driving voltage.
When a positive signal is applied to the input terminal 8, transistor 6 turns on and the positive voltage is supplied to the gate of the IGBT 1 from the power source 4, resulting in turning-on of the IGBT 1.
On the other hand, when a negative signal is applied to the input terminal 8, negative voltage is supplied to the gate of the IGBT 1 to turn it off.
In this case, the relation between ON voltage, that is a collector to emitter voltage V.sub.CE when the IGBT is in the ON state, and the ON current, that is, a collector current I.sub.C when the IGBT is in the ON state, is shown in the graph of FIG. 2. The relation between the collector to emitter voltage and the maximum collector current I.sub.CMAX is shown in FIG. 3.
It will be seen from FIG. 2 that the IGBT has intermediate characteristics between transistor and thyristor, that is, it shows constant current characteristics similar to transistors in low gate voltage areas and low voltage drop in high gate voltage areas.
It is apparent that if the IGBT is driven at a higher gate voltage V.sub.GE and the lower is the ON voltage V.sub.CE, thus the power loss of the IGBT 1 is decreased.
However, there is a problem that the IGBT often breaks down due to overcurrent within the range as shown by hatching lines in FIG. 3, when a shortcircuit accident of the load 3 in FIG. 1 occurs, because the voltage of d.c. power source is directly applied across the collector and emitter.
Considering such accident, if the IGBT is driven at a low gate voltage V.sub.GE, there occurs another problem that the power loss increases because the voltage V.sub.CE increases.
FIG. 4 shows the switching characteristics of an IGBT, in which collector to emitter voltage V.sub.CE begins to drop with a time delay Td after turning the gate voltage V.sub.GE from negative to positive, and it drops below 10 V at Tf. For a high speed IGBT, Td is approx. 0.5 .mu.s and Tf is approx. 1 .mu.s.
Thus, it is difficult to determine whether a shortcircuit accident occurs or not unless it is determined whether the high collector to emitter voltage is caused by either overcurrent or transient phenomena, since the turn-on operation has the above-mentioned delay in relation to the gate voltage V.sub.GE.
In the case of some types of IGBT, a direct shortcircuit is prohibited.
As to such type of IGBT, IGT (Insulated Gate Transistor) of GE corporation is known and introduced in, for example, "Application of Insulated Gate Transistors" (Factory Electronics, 1983).
FIG. 5 shows the safe operation area (SOA) of such IGBT and it is understood that, for example, the element having gate to emitter resistance R.sub.GE =5k can always be operated safely if the collector current is limited to 20 A or less.
However, if a current larger than the rated maximum current is turned off, the so-called latch-up phenomena occurs and the collector current I.sub.C cannot be made to 0 when the gate voltage V.sub.GE is made to 0. Moreover the element will be damaged because of an increase in current density in the element. Therefore it is necessary to prevent the IGBT operation beyond the SOA.